1. Field of the Invention
The present invention relates to a power line communications system using an electric power line as a carrier line.
2. Description of the Related Art
In recent years, there is known a power line communications system using an electric power line as a carrier line (see e.g. Japanese Unexamined Patent Publication No. 2004-56766). There is also known a power line communications system having the following arrangement. FIG. 12 is a diagram schematically showing an arrangement of the conventional power line communications system. The power line communications system shown in FIG. 12 includes slave apparatuses 100, a master apparatus 200, couplers 300-1 through 300-(n-1), transformers 400, and breakers 500. The slave apparatuses 100 are individually connected to electric power lines L100-1 through L100-n. The electric power lines L100-1 through L100-n are connected to the respective corresponding transformers 400 for dropping an electric power voltage from a commercial power source. The couplers 300-1 through 300-(n-1) are cascade connected. The electric power line L100-1 is connected to the master apparatus 200 via the corresponding breaker 500. The electric power lines L100-2 through L100-n are connected to the master apparatus 200 via the respective corresponding breakers 500 and the respective corresponding couplers 300-1 through 300-(n-1). The power line communications system is configured in such a manner that the master apparatus 200 is operable to collect information in the slave apparatuses 100 by power line communications. Hereinafter, the couplers 300-1 through 300-(n-1) are called as couplers 300 when generically referred to, and the electric power lines L100-1 through L100-n are called as electric power lines L100 when generically referred to.
Specifically, the power line communications system is used in e.g. a system for remotely reading electric power meters equipped in a housing complex. In the power line communications system, the master apparatus 200 and the couplers 300-1 through 300-(n-1) are installed in an electric room or a switch room of an architectural structure such as a building or an apartment house. The slave apparatuses 100 are individually installed in the electric power meters equipped in the houses. The master apparatus 200 collectively gathers information on indicator values of the electric power meters in the houses via the electric power lines L100-1 through L100-n, without individually reading the electric power meters on-site.
FIG. 13A is a block diagram showing an arrangement of the master apparatus 200. The master apparatus 200 includes a communicator 201, a power line connecting terminal device 202, a communication connector 203, a coupler connecting terminal device 204, a controller 205, and a power supply section 206.
The communicator 201 has a PLC (Power Line Communications) modem, and performs power line communications with the slave apparatuses 100 via the electric power lines L100-1 through L100-n. The power line connecting terminal device 202 is connected to the electric power line L100-1 along which a commercial electric power from a commercial power source is supplied to the master apparatus 200. The communication connector 203 is connected between the communicator 201 and the power line connecting terminal device 202 to cut off a frequency component of commercial power supply, and transmit a frequency component for use in power line communications. The coupler connecting terminal device 204 is connected to a connecting terminal device 303 in the corresponding coupler 300 via a corresponding electric power line L101.
The controller 205 includes a microcomputer, and controls overall operations of the master apparatus 200. The power supply section 206 includes a power supply circuit for rectifying and smoothing the commercial electric power supplied from the power line connecting terminal device 202 to generate an electric power for driving the controller 205 and the communicator 201.
FIG. 13B is a block diagram showing an arrangement of the coupler 300. The coupler 300 includes a communication connector 301, a slave apparatus connecting terminal device 302, and two connecting terminal devices i.e. the aforementioned connecting terminal device 303 and another connecting terminal device 304. The communication connector 301 is adapted to cut off a frequency component of commercial power supply, and has four capacitors C for transmitting a frequency component for use in power line communications, and a connecting transformer 311. The slave apparatus connecting terminal device 302 is connected to the corresponding slave apparatus 100 via the electric power line L100. The connecting terminal device 303 is connected to the coupler connecting terminal device 204 in the master apparatus 200 or the other coupler 300 via the corresponding electric power line L101. The connecting terminal device 304 is connected to the other coupler 300 via the electric power line L101 shown in FIG. 12.
In the following, an operation of the conventional power line communications system shown in FIGS. 12, 13A, and 13B is described. When a communication signal requesting data transmission to the slave apparatuses 100 is outputted from the communicator 201, the communication signal is transmitted to the corresponding slave apparatuses 100 via the communication connector 203, the power line connecting terminal device 202, and the electric power line L100-1, and likewise, transmitted to the corresponding slave apparatuses 100 via the communication connector 203, the coupler connecting terminal device 204, the corresponding electric power lines L101, the corresponding couplers 300, and the electric power lines L100-2 through L100-n.
Upon receiving the communication signal, each of the slave apparatuses 100 transmits, to the master apparatus 200, a communication signal including data of its own (e.g. an indicator value of the electric power meter). The communication signal transmitted from the corresponding slave apparatuses 100 connected to the electric power line L100-1 is inputted to the communicator 201 via the power line connecting terminal device 202 and the communication connector 203. The communication signal transmitted from the corresponding slave apparatuses 100 connected to the electric power lines L100-2 through L100- n is inputted to the communicator 201 via the coupler connecting terminal device 204 and the communication connector 203. In this way, the master apparatus 200 collects the information on the indicator values of the electric power meters from the slave apparatuses 100.
In the conventional power line communications system, the coupler connecting terminal device 204 is connected between the power line connecting terminal device 202 and the communication connector 203. Accordingly, a voltage of e.g. AC 200V, whose frequency component of commercial power supply is not cut off, may be applied from the communication connector 203 to the electric power lines L101 for connecting the master apparatus 200 and the corresponding couplers 300.
Accordingly, in the case where live-line maintenance and repair work is performed by replacing the coupler 300 or temporarily disconnecting the coupler 300 from the power line communications system, without cutting off the commercial electric power to be supplied to the master apparatus 200, an operator may be brought to a hazardous electric shock condition during the work. In view of this, the above operation is required to be performed by cutting off the commercial electric power to be supplied to the master apparatus 200, which may obstruct communications between the master apparatus 200 and the slave apparatuses 100 during the operation.